A phoneme is a basic unit of a language's phonology, which is combined with other phonemes to form meaningful units such as words or morphemes. The phoneme can be described as "the smallest segmental unit of sound employed to form meaningful contrasts between utterances". In this way the difference in meaning between the English words kill and kiss is a result of the exchange of the phoneme /l/ for the phoneme /s/. Two words that differ in meaning through a contrast of a single phoneme are called minimal pairs.
Within linguistics there are differing views as to exactly what phonemes are and how a given language should be analyzed in phonemic terms. However, a phoneme is generally regarded as an abstraction of a set (or equivalence class) of speech sounds (phones) which are perceived as equivalent to each other in a given language. For example, in English, the "k" sounds in the words kit and skill are not identical (as described below), but they are distributional variants of a single phoneme, /k/. Different speech sounds representing the same phoneme are known as allophones, and such variation may be conditioned, in which case a certain phoneme is realized as a certain allophone in particular phonological environments, or it may be free in which case it may vary randomly. In this way, phonemes are often considered to constitute an abstract underlying representation for words, while speech sounds make up the corresponding phonetic realization, or surface form.
Phonemes are conventionally placed between slashes in transcription, whereas speech sounds (phones) are placed between square brackets. Thus /pʊʃ/ represents a sequence of three phonemes /p/, /ʊ/, /ʃ/ (the word push in standard English), while [pʰʊʃ] represents the phonetic sequence of sounds [pʰ] (aspirated "p"), [ʊ], [ʃ] (the usual pronunciation of push).
(Another similar convention is the use of angle brackets to enclose the units of orthography, namely graphemes; for example, ⟨f⟩ represents the written letter (grapheme) f.)
The symbols used for particular phonemes are often taken from the International Phonetic Alphabet (IPA), the same set of symbols that are most commonly used for phones. (For computer typing purposes, systems such as X-SAMPA and Kirshenbaum exist to represent IPA symbols in plain text.) However descriptions of particular languages may use different conventional symbols to represent the phonemes of those languages. For languages whose writing systems employ the phonemic principle, ordinary letters may be used to denote phonemes, although this approach is often hampered by the complexity of the relationship between orthography and pronunciation (see Correspondence between letters and phonemes below).
Other features with phonemic status
While phonemes are normally conceived of as abstractions of discrete segmental speech sounds (vowels and consonants), there are other features of pronunciation – principally tone and stress – which in some languages can change the meaning of words in the way that phoneme contrasts do, and are consequently called phonemic features of those languages.
Phonemic stress is encountered in languages such as English. For example, the word invite stressed on the second syllable is a verb, but when stressed on the first syllable (without changing any of the individual sounds) it becomes a noun. The position of the stress in the word affects the meaning, and therefore a full phonemic specification (providing enough detail to enable the word to be pronounced unambiguously) would include indication of the position of the stress: /ɪnˈvaɪt/ for the verb, /ˈɪnvaɪt/ for the noun. In other languages, such as French, word stress cannot have this function (its position is generally predictable) and is therefore not phonemic (and is not usually indicated in dictionaries).
Phonemic tones are found in languages such as Mandarin Chinese, in which a given syllable can have five different tonal pronunciations. For example, mā (high level pitch) means "mom", má (rising pitch) means "hemp", mǎ (falling then rising) means "horse", mà (falling) means "scold", and ma (neutral tone) is an interrogative particle. The tone "phonemes" in such languages are sometimes called tonemes. Languages such as English do not have phonemic tone, although they use intonation for functions such as emphasis and attitude.